Effect of Sn on CO oxidation over Rh–Sn/SiO2 bimetallic catalysts

Abstract

Oxidation of CO by O₂ has been carried out over Rh/SiO₂ and Rh–Sn/SiO₂ catalysts. The temperature at which the activity for CO oxidation appeared was lowered in the presence of Sn. The reaction kinetics were not affected by the addition of Sn, the adsorption capacity for H₂ and CO decreased, whereas the capacity for O₂ increased. Sn enhanced dissociative adsorption of molecular oxygen. Successive adsorption of CO and O₂ indicated that the preadsorption of CO or O₂ did not decrease the adsorption capacity of Rh–Sn/SiO₂ for O₂ and CO, respectively. On the other hand, strong inhibition of the adsorption capacity was observed on Rh/SiO₂ after preadsorption. Temperature-programmed desorption of adsorbed CO and co-adsorbed CO–O₂ showed that the chemisorption of CO on Rh was weakened by the addition of Sn and that a larger amount of CO₂(product of CO oxidation reaction) was desorbed from Rh–Sn/SiO₂ than from Rh/SiO₂. The surface reaction between adsorbed CO and O was enhanced by the addition of Sn. The role of Sn in Rh–Sn/SiO₂ for CO oxidation is considered as: (1) to enhance the activation of oxygen molecules, (2) to decrease the self-poisoning by CO and/or O₂, (3) to enhance the surface reaction between adsorbed oxygen and weakly adsorbed CO.